The present invention relates in general to digital amplifiers, and in particular, to delta-sigma amplifiers with supply voltage variation compensation and methods and digital amplifier systems using the same.
Delta-sigma modulators (noise shapers) are particularly useful in digital to analog and analog to digital converters (DACs and ADCs). Using oversampling, a delta-sigma modulator spreads quantization noise power across the oversampling frequency band, which is typically much greater than the input signal bandwidth. Additionally, a delta sigma modulator performs noise shaping by acting as a lowpass filter to the input signal and a highpass filter to the noise; most of the quantization noise power is thereby shifted out of the signal band.
In addition to data conversion applications, delta-sigma noise shapers are increasingly utilized in the design of digital amplifiers. In one particular technique, a digital delta-sigma noise shaper provides a noise shaped (quantized) digital data stream to a pulse width (duty cycle) modulator PWM, which in turn drives a linear amplifier output stage and associated load. This technique is generally described in U.S. Pat. No. 5,784,017 entitled xe2x80x9cAnalogue and Digital Convertors Using Pulse Edge Modulators with Non-linearity Error Correctionxe2x80x9d granted Jul. 21, 1998 and U.S. Pat. No. 5,548,286 entitled xe2x80x9cAnalogue and Digital Convertor Using Pulse Edge Modulators with Non-linearity Error Correctionxe2x80x9d granted Aug. 20, 1996, both to Craven, U.S. Pat. No. 5,815,102 entitled xe2x80x9cDelta Sigma PWM DAC to Reduce Switchingxe2x80x9d granted Sep. 29, 1998 to the present inventor (incorporated herein by reference), U.S. patent application Ser. No. 09/163,235 to the present inventor (incorporated herein by reference), and International Patent Application. No. PCT/DK97/00133 by Risbo.
One difficulty in implementing these digital amplifiers is minimizing noise and distortion due to power supply noise and variations. This problem is correctly identified in U.S. Pat. No. 5,559,467 to Smedly (xe2x80x9cthe ""467 patentxe2x80x9d). Specifically, the ""467 patent recognizes the need to account for the time-varying value of the power supply voltage during modulation; however, the solution proposed in the ""467 patent introduces its own distortion into the system.
Hence, improved circuits and methods are required for minimizing noise and distortion in digital amplifiers in light of power supply noise and time variations.
The principles of the present invention allow for the measurement of the sum and difference between the supply voltages supplying an amplifier output stage. The measured sum and difference are then utilized by a noise shaper driving the input to the output stage to compensate for variations in the supply voltages. When applied to ADC circuits and systems, these principles advantageously provide an output signal which is less sensitive to power supply noise and variations.
According to one particular embodiment, a delta-sigma modulator is disclosed for driving an output stage operating between first and second voltages including a loop filter, a quantizer, and a feedback loop coupling an output of the quantizer and an input of the loop filter. The feedback loop includes compensation circuitry for compensating for variations in the first and second voltages in response to a measured average of the first and second voltages and a measured difference between the first and second voltages. Measuring circuitry measures the average and the difference of the first and second voltages.
Advantageously, the principles of the present invention allow for amplifier supply voltage variations to be corrected in the noise shaper stage without introducing other noise and distortion into the amplifier output signal. These principles are particularly useful in digital amplifiers, such as digital audio amplifiers, and are applicable to a number of different amplifier output stages including half-bridge and full-bridge configurations. The output stages may be directly driven by the output of the noise shaper or through an intermediate stage such as a PWM converter.